Sound reproducing system



Oct. 31, 1944. W At MUELLER SOUND REPRODUCING SYSTEM Filed Deo.

' to the other.

Patented Oct. 31, 1.9444

I UNITED STATES PATENT OFFICE SOUND REPRODUCING SYSTEM William A. Mueller, Los Angeles, Calif., assignor to Radio Corporation of America, a corporation of Delaware i Application December 29, 1941, Serial No. 424,749

' 13 claims. (C1. 17a-100,3)

This invention relates to sound reproducing systems and particularly to a theatre sound system employed in conjunction with the projection of motion pictures.

During recent years, the exhibition of motion pictures with sound has improved considerably,

' one of these improvements being in the reproducmay be expanded beyond that on the film by the use of independent control tracks or records, or such control tracks may be used to considerably increase the volume level of only thex normally.

high level passages.

Another function of control tracks is to distribute the sound among different speakers locatedacross the proscenium of a theatre in accordance with the nature of the sound source being depicted at any particular time. -For init is necessary to switch back and forth between the center speakers operating together with the side speakers and the center speakers operating alone.

The present invention, therefore, is directed to a system wherein the changeover from center toV center and side speakers and vice versa is accomplished in such a manner that the listeners are not conscious of the changefrom one condition It is well known that when a single loudspeaker is receiving a certain amount of energy and that energy is then divided between two speakers, the reduction in acoustic output from the ilrrst speaker is substantially one-half or, in the terms of input energy, substantially 3 db. Thus, if the speakers were separated and a listener were positioned Vdirectly infront of one speaker, the changeover from'one speaker to both fore, is to facilitate and improve the reproduction and propagation of sound.

Another object of the invention is to vary the position or 'positions of sound sources with re'- spect to a listening area without substantially varying-the acoustic. output for a constant signal level.

A further object of the invention is to main tain substantially constant the acoustic energy within a certain listening area during variations in theposition or positions of the source of propagation oftheacoustic energy.

A further object of the invention is to alternately centralize and spread the propagation ofsound and automatically maintain a constant acoustic level Within a given listening area. I

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the appended claims, the manner of its organization and the mode of itsyoperation will be better understood byreferring to the following description read in conjunction with the accompanying drawing forming a part thereof, in which l Fig. 1 is a diagrammatic arrangement of a general system involving the invention'f and Fig. 2 is a combination schematic-diagrammatic arrangement of a particular type of theatre sound reproducing circuit involving the iny a variable gain amplier 8 and also controlling speakers would be quite noticeable. This probthe point of switchover between center and sid speakers. The output of the variable gain amplifier is fed into a power amplifier 9 and th`en to a center speaker' I0. When a switch I4 is closed by the vcontrol track action, side speakers Il and I2 are paralleled with the center speaker IIJ. This is the general arrangement wherein the soundhead 5 reproduces signal and control tracks,

the control track varying the amplitude of the may operate solely on dialogue, while any increased sound level or particular type of sound, such as that from large orchestras or other sound effects of avalanches or storms, will be divided among the three speakers I Il, l-I and I2 to entransmitted to the speakers in any particular, grouping .as disclosed and claimed in H. J. Reiskind Patent No.' 2,299,410 of .October 20, 1942.

and U. s. Patent No. 2. 14.382 of M. c. Batsei o! March 23. i943. It realized that when changeover occurs .in one direction, the energy' from the power ampliiier l ls divided among the three speakers and that a listener directly in vfront of the center speaker would ordinarily notice a decrease in volume at the changeover point.

The .present invention minimizes this decrease by automatically varying the transmission loss be- I tween the power amplilier and the speakers in accordance withthe number of speakers connected to the amplier.

Reierring now to Fig. 2, a` lm I is shown having a signal track 2i and a control track 22, the latter being located in the sprocket hole reglon of theiilm as disclosed and claimed in U. S. Patent No. 2,070,260 of C. M. Burrill, of January '20, 1942. An optical system for the signal recupon a pre-ampliiier 32 and then passed through a voltage ampliiler 33 and variable gain ampliiler 35 to a power amp er Il. The output of the photoelectric cell I isimpressed upon a detector amplier 36 and then on a control unit 3l,

as disclosed in the above-mentioned patent of C. M. Burrill. Control unit 31 varies the output of .the variable gain ampliiler 35 in accordance with the variations of control track 22 and also, at a definite cue or level, operates a single contact switch l5 in any suitable mannersuch as shown in the above-mentioned M. C. Batsel application.

The output of the power amplier 3l is impressed upon a crossover'network 4i comprising variable inductances l2 and l5 and variable capacitances u and l5, these variable elements' being adjusted to a definite value in accordance with the frequency bands to be reproduced by low-frequency and high-frequency speakers, as is well known in the art.- Connected tothe crossover network is an auto-transformer 41,.which feeds four center low-frequency Speakers 5I, 8|,

the pnly load on amplier 34 when-the center speakers 50, 5l, 52, 53, 55 and 55-are operating assumo installation ci speakers in accordance with the invention wherein the center and side speakers compose a plurality of units, although the invention is applicable to a system using any number ot speaker units, as will be understood from the following expl on.

As shown in 2, the output impedance of the power am er Il is 'll/2 ohms and is connected to the l5-ohm network 4I only when the switch 3l is open. this network constituting alone. It will be noted a l-to-2 impedance mismatch exists with this connection. However, with the'swltch 35 closed, the two crosover network 42- and 55 are in parallelwith the power` ampliiler and the output impedance of the power ampliileris now matched to its load. Thus, impedances are matched when the power ampliiier is feeding both center `and side speaker units, but

mismatched when only the center speaker is op erating. A

It is well known that to obtain the maximum power transfer from a generator to a load or from one unit to another, the impedance of the two units connected should be matched and that any mismatch ordinarily inserts a transmission loss. This principle is utilized in the present. invention. I nY this circuit, however, it will be noted that the transmission loss is automatically inserted between the power amplier and the network L2 when the switch 39 is open, which corresponds to when onlythe center speakers are operating. Thus, in this condition, the center speaker units will receive a certain energy from the-power ampliiier 34 for a given input energy to the latter. Now, when the switch 39 is closed and the side speakers are connected to the power amplifier, the change in the impedance match is 40 in a direction to increase the energy passing to 52 and 53, and also an auto-transformer 48 which l .Vwork 58 having inductancesa and 65; similar to inductances I2 and I3 of network 4I and variable capacitances, 6I and 52 similar'to capacitances u and l5. The output of the -net- 'wo'rk'58 is fed into an auto-transformer .65, to which'.A are connected two low-frequency side speakers 61 and 6I positioned on the 'left of the center speakers, and two low-frequency speakgers 'Il and 1I positioned onthe'right of the center speakers. To another auto-transformer 6l areconnected a highfrequency speaker 1l.asso ciated with the side Speakers 51 and 5l, and a high-frequency speaker 15 associated withthe low-frequency' speakers Il'and 1I. Y

'I'he above description-is ci a.

hound waves, and means for connecting oneor all the speakers, thus compensating for the drop in energy from the center speakers caused by the side speakers being `connected.

From measurements made in actual practice,

was found to be in the neighborhood of 0.8 o!V a db. instead of the normal 3 db.. which would occur with matched impedances under both conditions. The result of this arrangement is that the listeners directly in'front of the center speakers have diiliculty in detecting any change whatsoever in the output level. Y In fact, the spreading eilect from the side speakers apparently compensates for this small decrease in volume from the center units. Thus, it has been found that switching back and forth between 'the center speaker and the center speaker and side speakers combined 1s notonceable to che listening susience, while obtaining the dramatic eiects of the spread sound source. Although certain deilnite values of. impedances havebeen given in the above description, it is to be understood that other values `will function in the same manner andA produce the same .results if the ratios and the direction of the impedance changes are in accordance vwith this invention." 1 What I claim as my invention is;-

l. A sound reproducing system comprising a` source of signal currents to be reproduced, an amplifier for-'said currents, a plurality of sound reproducers for translating-said currents into more ot said reproducers to the cutput'clrcuit ci said amplifier, said combined impedances ci said vreproduces-s and impedance ot theoutput cir.

the actual decrease in input to the center speaker lall of said reproducers are T3connected to sai cuit f said amplifier being substantially matched when all of said reproducers are connected to said output circuit, and saidq impedances being mlsmatchedwhen` less tharrall of said reproducers are connected to said output circuit, said impeduce variationmaintaining the acoustic output of any originally connected reproducer substantially constant for an., input signal of any given level when one or more additional-reproducers are connected t/othe output circuit of said amplifier.

2. A sound reproducing system in accordance with c1aim1 in which the connection of additional groups-sof reproducers to said output circuit is varied by a control track at a certain predetermined level in said originally connected reproducers. f

3. A reproducing system in accordance with claim 1 in which a variable gain amplifier is provided for varying the ratios between the amplitudes of said currents as reproduced and the amplitudes of said currents at said source, said trol record.'

4. A system for the reproduction of sound by a variable plurality of reproducing units comprising means for generating signal currents corresponding to sound waves, means for amplifying said currents;| and means for connecting the output 'circuit of saidl amplier with said reproducers, said last-mentioned means being adapted to vary the number of reproducers actually connected to the output circuit of said ampliiier at any particular time, the impedances of said reproducers having such a relationship with respect to the impedance of said output circuit that a mismatch in impedances is provided between the output circuit of said amplifier and one of said reproducers, and matched impedances are provided ,between the output circuit of said amplier and al1 of said reproducers, said impedance variation maintaining the acoustic output of any connected reproducer substantially constant for an input signal of any given level when one or more additionalreproducers are connected or disconnected to the output circuit of said ampliiier. 5. A system for the reproduction of sound in accordance with claim 4 in which means are provariable gain amplifier being controlled by a conof amplification of said signal currents, and the number of reproducer units actually connected to I said output circuit 'at any particular time.

6. A system forthe reproduction of sound in accordance with claim 4 in which said signal currents and a secondary current for controlling the ratio of ampllcation of said signal currents are obtained from photographic lm records.

7. I'he method of maintaining a substantially constantacoustic output from asound reproducer adapted to be actually connected to and disconnected from ay signal amplier aioiig with other reproducers comprising selecting the impedance of each of said reproducers and the impedancepf said. amplifier to have a relationship s uch that are connected to said amplifier, the impedance is mismatched,"sald impedance variation intainving the acoustic output of any connected repro'- the combined impedance of all of said reproducersvv ducer substantially constant for an input signal yof a certain level during the connection and ldisconnection of other reproducers to said amplifier.

8. The method in accordance with claim 7-in which the impedance of the reproducers is greater than the impedance of the ampliiier during the periods when an impedance mismatch exists between said amplier and said reproducers connected thereto.

9. A system of sound reproduction with a plurality of sound reproducers adapted to be connected and disconnected duringsound reproduction comprising means for generating signal currents, means for amplifying said currents, a plurality of reproducers having respective associated units for high and low frequency components of said signal currents, and a plurality of crossover networks connected intermediate said units and the output circuit of said amplifying means, the impedances of said crossover network being matched to the respective units connected thereto and matched to said output circuit when all of' said crossover networks are connected to said output circuit, but the impedance of said output circuit being mismatched with the impedance of any one of said crossover networks.

10. A system of sound reproductionin accordance with claim 9 in which said signal current generating means includes a sound film record,

and means are provided for generating control currents from said sound lm for varying the numbers of said reproducing units being connected to and disconnected from the same output circuit as sani opei ating units by actuation ol' said switching means at a predetermined acoustic output level from said operating units, the input impedance to said reproducing units having a relationship to the output impedance of said output circuitsuch that -a mismatch of impedances is' obtained between said output circuit and said reproducing units when less than all of said repro-i ducing units are connected to said output circuit anda matching of impedances is obtained between said output circuit andv said reproducing units when aliof said reproducing units are connected to said output circuit. l

12. A system in accordance with claimll in which said switching means includes a relay having contacts connected to saidoutput circuit and contacts connected to certain of said reproducing 13. AV system in accordance with claim 11 in which a second means for generating control cur- ]rentsjs provided, said control currents actuating .said switching means to connect and disconnect said additional units toand from .saidoutpu't circuit.' WILLIAM A. MUELLER. 

